The plant hormone auxin is involved in virtually all aspects of plant growth and development. During the last grant period we demonstrated that auxin acts by stimulating the degradation of a family of transcriptional repressers called the AUX/IAA proteins, a process that requires the ubiquitin protein ligase SCFTIR1 and related SCFs. In our most recent studies we have shown that auxin interacts directly with SCFTIR1 or a closely associated protein. We propose that auxin binding promotes the interaction between SCFTIR1 and its' substrates, the AUX/IAA proteins. This is a novel mechanism of hormone action and SCF regulation. The current proposal has three specific aims. The first is to investigate auxin regulation of AUX/IAA degradation. The region of TIR1 that is required for AUX/IAA recognition will be identified. Biochemical experiments will be conducted to identify and characterize proteins that mediate AUX/IAA recognition and in particular the direct target of auxin action will be defined. Further, the structure of the AUX/IAA proteins will be determined in collaboration with my colleague, Dr. L. Chen. In the second aim we will characterize the Arabidopsis root transcriptome. The goals of this aim are to identify downstream targets of AUX/IAA regulation and to compare and contrast auxin regulated transcription in cell types with different auxin growth responses. A number of candidate auxin-response genes have been identified and these will be characterized. In the third aim, we will conduct novel genetic screens for new mutants deficient in auxin response. Our emphasis will be on mutants with severe defects in embryo and root development that are unlikely to be recovered in previous screens. We expect our experiments to provide fundamental new insight into the mechanism of auxin action. Further, our studies indicate that auxin acts through a novel mechanism that involves direct binding to a ubiquitin protein ligase. These results will have significant impact on studies of protein degradation in diverse species including humans.